1. Field of the Invention
The present invention relates to a structure of a display apparatus using thin-film transistors that are formed by making use of a crystalline semiconductor thin film that is formed on an insulating substrate.
2. Description of the Related Art
An active-matrix flat-panel display using thin-film transistors is known as a display apparatus for OA equipment, etc., which displays image information and character information. In recent years, with a development of multi-media communication technology, a function-integrated display that is called “system-on-panel” has attracted attention as a next-generation display. This new type of display has a small size and light weight for personal use, and has high resolution and high image quality. In addition, peripheral functional components, such as driver circuits, a memory circuit, a DA converter circuit and an image processing circuit, which relate to image display, are integrated on the display panel.
This display apparatus for use in personal computers, etc., however, adopts a display scheme wherein an image signal, which is obtained by decompressing compressed data, is received and displayed. To be more specific, a system apparatus such as a personal computer, which is constructed separately from the display panel, decompresses compressed data that relates to an image to be displayed, thereby generating bitmap data. The bitmap data is stored in a video memory within the system apparatus. The bitmap data corresponding to each display bit, which is stored in the video memory, is successively transmitted from an LCD (liquid crystal display) controller within the system apparatus to the display apparatus. On the display apparatus side, the transmitted data is latched on a line-by-line basis, and the data is output from a data driver circuit. Thus, an image is displayed on a line-by-line basis.
FIG. 1 shows an example of a liquid crystal display apparatus, and FIG. 2 is a cross-sectional view thereof. A display apparatus 100 includes a pair of transparent substrates 121 and 122 formed of, e.g. glass plates, a liquid crystal layer 123, a plurality of pixel electrodes 124, scan lines 125, signal lines 126, a counter-electrode 127, and a plurality of thin-film transistors 130. As is shown in the cross-sectional view of FIG. 2, the liquid crystal layer 123 is disposed between the paired transparent substrates 121 and 122. The plural pixel electrodes 124, which are disposed in a matrix in row and column directions, and the plural thin-film transistors 130, which are connected to the associated pixel electrodes 124, are provided on one of the transparent substrates, 122. Each pixel electrode 124 and each thin-film transistor 130 are connected to the associated scan line 125 and signal line 126. Each scan line 125 is connected to a scan line drive circuit 141, and each signal line 126 is connected to a signal line drive circuit 142. The scan line drive circuit 141 and signal line drive circuit 142, which are formed on the substrate 122, are connected to the LCD controller 143 within the system apparatus 145, which is connected to the display apparatus 100 via a transmission line 144 that is formed of a signal cable. The scan line drive circuit 141 receives a vertical scan control signal YCT from the LCD controller 143, and the signal line drive circuit 142 receives a pixel video signal Vpix as bitmap data, and a vertical scan control signal XCT from the LCD controller 143.
In the above-described structure, if the definition and the number of colors of an image to be displayed increase, the amount of transmission data, such as pixel video signals Vpix, increase accordingly. On the other hand, since the refresh rate for image display is constant, the clock frequency of the transmission line 144, which is formed of the signal cable, needs to be increased in accordance with the increase in amount of data. If the frequency of the transmission line 144 increases, there arises such a problem that undesirable electromagnetic radiation occurs from the transmission line and noise is produced in the external apparatus due to electromagnetic interference (EMI). To solve this problem, a technique of reducing EMI by differential driving with low voltage, which is known as LVDS (Low Voltage Differential Signaling), has been adopted. An example of this technique is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-176350.
In the above-described structure wherein the display apparatus 100 and the LCD controller 143 that is provided in the system apparatus 145 are connected via the transmission line 144 that is composed of the signal cable, special consideration needs to be paid to the design of the transmission line in order to reduce EMI from the transmission line. In a desktop computer, for instance, the LCD controller 143 and display apparatus 100 need to be connected by a dedicated shield cable that is designed to prevent unnecessary radiation. In a notebook computer, the display apparatus 100 and LCD controller 143 are disposed in a single casing and they are connected by a short flexible cable. That is, in this technique, it is necessary that the transmission line between the LCD controller 143 and display apparatus 100 be designed to have a data transmission rate enough to transmit image data that is to be displayed. Hence, in order to display an image on the display apparatus 100, there is no choice but to dispose the LCD controller 143 near the display apparatus 100. It is thus difficult to use the computer in such a way that the light-weight display apparatus alone is moved and made to display an image at a place where image display needs to be performed.
This type of display apparatus, however, is not adaptable to such a future ubiquitous computing environment that computers are used anywhere, anytime. The anywhere, anytime use means that the transmission line with a sufficient performance is not always secured.
As stated above, there is such a problem that the display apparatus 100 that requires a very high clock frequency for transmitting bitmap data is not easily adaptable to the environment, such as a wireless transmission system, where a sufficient transmission capacity is not always secured.
An object of the present invention is to provide a display apparatus wherein, for example, compressed image information, which is to be decompressed subsequently, can be transmitted to the display apparatus in such an environment where a sufficient transmission capacity is not always secured, and good image display is enabled.
Another object of the present invention is to provide a structure of a thin-film transistor for realizing a display apparatus that meets the above demand at low cost.